1. Field of the Invention
The present invention relates to a fixing device for use in an image forming apparatus of an electrophotographic system, an electrostatic recording system and the like, and an image forming apparatus including the fixing device.
2. Description of Related Art
Generally, an image forming apparatus of the electrophotographic system (such as a printer, copier, and facsimile machine) has a fixing device that applies heat and pressure to a sheet on which a toner image has been transferred to fix the toner. Such a fixing device includes a heating section that heats toner borne on a sheet to melt the toner, and a pressing section that presses the sheet against the heating section.
The pressing section of the fixing device includes, for example, a fixing roller and a pressure roller that is pressed against the fixing roller with a predetermined load. The pressure roller is pressed directly or indirectly against the fixing roller, thus forming a nip portion for conveying a sheet in a sandwiching manner.
The heating section of the fixing device is composed of, for example, an endless fixing belt provided around a heating roller having a heating source (for example halogen heater) therein and the fixing roller in a stretched state (heat belt type). In this case, the pressure roller is pressed against the fixing roller with the fixing belt therebetween, thus forming the nip portion. In addition, the fixing roller may have the heating source therein and the fixing roller itself may serve as the heating section (heat roller type). In this case, the pressure roller is pressed directly against the fixing roller, thus forming the nip portion.
In an image forming apparatus having the above-described fixing device, a toner image is developed on a photoconductor drum based on image data, and the toner image thus developed is transferred onto a sheet. Then, the sheet on which the toner image has been transferred is conveyed to the fixing device, and heat and pressure is applied to the sheet at the time when the sheet passes through the nip portion, whereby the toner image is fixed to the sheet.
Such a fixing device is known as disclosed in Japanese Patent Application Laid-Open Nos. 6-250560, 10-221999, and 9-138598, for example.
In the fixing device of Japanese Patent Application Laid-Open Nos. 6-250560 and 10-221999, part of an endless belt provided around a plurality of rollers in a stretched state is wound around a fixing roller so as to form a nip portion. A pressure roller, which makes pressure contact with the fixing roller from the internal circumference side of the endless belt with the endless belt therebetween, is provided at an exit portion of the nip portion. In the fixing device, a braking force is exerted on the endless belt during conveyance in a region of the pressure roller making pressure contact with the fixing roller so as to equal the difference in velocity between the pressure roller and the fixing roller, thereby preventing image deviation. Meanwhile, in the fixing device of Japanese Patent Application Laid-Open No. 9-138598, a heat-resistant belt is supported by a plurality of rollers around which it is wound, and a pressure roller is pressure contact with the rollers with the heat-resistant belt therebetween. In addition, the rollers are braked to impart a tensile force to the heat-resistant belt.
Incidentally, in the fixing device, a surface of a sheet on which an unfixed toner image is borne directly contacts with the heating section at a fixing step (fixing belt or fixing roller). At this time, occasionally, wax exuded from toner adheres to the heating section (fixing belt or fixing roller), and a latent image is formed by the wax adhered to the heating section and visualized in the next image. To be more specific, the wax adhered to the heating section is visualized by a phenomenon (referred to as gloss memory) in which the wax adhered to the heating section is visualized in the form of gloss unevenness caused by a portion having a small amount of the adhered wax and a portion having a large amount of the adhered wax when toner for forming the next image is fixed.
There has been a desire to eliminate the gloss memory at the time of fixing toner in the fixing device to improve image quality. It is to be noted that the problem of prevention of the gloss memory cannot be solved by the fixing devices disclosed in Japanese Patent Application Laid-Open Nos. 6-250560, 10-221999, and 9-138598.